This is when you wager on all your chips on the table. When the player goes all-in, he gets special treatment since he is no longer able to make wagers or call on more wagers. You can not force this player to fold and will see the showdown without doubt. Active players on the table will finish the round normally
Especially at tournament poker, players are forced to play at the late game push or fold. That means to go all-in and risk all his chips or fold the hand. A lot of players push also first in any two all-in, if they are under pressure in a sit&go or multi table tournament.